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Pheromones and Chemical Communication in Lizards

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Pheromones and Chemical Communication in Lizards 43
CHAPTER
3
Pheromones and Chemical
Communication in Lizards
José Martín and Pilar López
3.1 INTRODUCTION
Pheromones have been defined, based on entomological studies, as
chemicals or semiochemicals produced by one individual that effect a
change in the physiology (‘primer’ pheromone) or behavior (‘releaser
pheromone) of conspecifi cs (Karlson and Lüscher 1959). In insects and many
other invertebrates, very often just one or a pair of chemical compounds
acts as an exclusive pheromone attracting the opposite sex. In contrast,
vertebrates often have multicomponent pheromones with a mixture of
many different chemical compounds with distinctly different functions
or intended receivers (Müller-Schwarze 2006). However, compounds may
be mixed together in specifi c proportions to determine “odor profi les”
of species or individuals (Johnston 2005; Wyatt 2010). The pattern of
compounds in the scent of an individual may convey various signals such
as sex, age, social status, group, individuality, seasonality, condition, health
state, etc. Moreover, in insects, pheromones alone can directly control
reproductive behavior, whereas in vertebrates, a combination of different
sensory stimuli (visual, tactile, chemical, etc.) is often required to control
reproduction. Therefore, in vertebrates, pheromones may be better defi ned
as a group of active compounds in a secretion that supply information to
conspecifi cs that may be relevant for reproductive decisions (for reviews
see Mason 1992; Wyatt 2003, 2010; Müller-Schwarze 2006; Apps 2013).
Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, José
Gutiérrez Abascal 2, 28006 Madrid, Spain.
44 Reproductive Biology and Phylogeny of Lizards and Tuatara
Reproductive behavior of lizards was traditionally thought to be
predominantly based on conspicuous visual signals, whereas the potential
role of pheromones in reproduction was not considered in most studies.
However, there was considerable evidence of the chemosensory abilities of
most lizards, and of the widespread occurrence of multiple types of glands
that secrete chemicals with the potential of being pheromones, especially
during the reproductive season. Only recently, has it been recognized
that in many species of lizards, pheromones (i.e., specifi c compounds or
mixtures of compounds) or “chemical signals” (i.e., undetermined chemical
secretions) are very important, and sometimes required, for species and sex
recognition, intrasexual relationships between males, social organization,
territorial marking, and mate choice of lizards (reviewed in Halpern 1992;
Mason 1992; Cooper 1994; Johansson and Jones 2007; Houck 2009; Mason
and Parker 2010; Martín and López 2011).
3.2 CHEMOSENSORY ABILITIES OF LIZARDS
Lizards, and most tetrapods, can use their chemosensory senses to detect and
discriminate many different scents in their environments coming from prey,
conspecifi cs and/or predators. These abilities are based on the possession
of higly developed olfactory and vomeronasal organs (Halpern 1992;
Mason 1992; Cooper 1994; Schwenk 1995; Halpern and Martínez-Marcos
2003). The olfactory and vomeronasal systems do not have independent
functions, as it was though in the past, but show deep anatomical and
functional interrelationships (Halpern and Martínez-Marcos 2003; Ubeda-
Banon et al. 2011). In many cases, scent stimuli are fi rst received through
the nares and processed by the nasal organs, and this triggers tongue-fl ick
mediated vomerolfaction (Halpern 1992; Cooper 1994; Schwenk 1995). The
vomeronasal organ sends specifi c chemical signals to the central nervous
system activating accessory olfactory pathways. In particular, chemical
compounds with a putative pheromonal function stimulate brain areas
involved in sexually dimorphic reproductive behavior.
Associated with chemoreception, tongue-fl icking (TF) is a characteristic
behavior of lizards and snakes in which the tongue is extruded to sample
chemicals from the environment that are delivered into the mouth and
transported to the vomeronasal organ (Schwenk 1995). This is an easily
observable and quantifi able behavior that has been used as a bioassay for
chemosensory discrimination abilities of lizards and snakes (Cooper and
Burghardt 1990; Cooper 1994, 1998). Different chemical stimuli impregnated
in cotton swabs, tiles or papers are randomly presented close to the snout
of experimental subject, and TF rates are measured during a certain time
period. Detection of a scent stimulus is inferred by an increase of TF rates
Pheromones and Chemical Communication in Lizards 45
in response to the presentation of a given scent above the baseline TF rates
observed under the experimental conditions in response to an odorless
control (e.g., deionized water). Differential TF rate to different scent stimuli
is considered as an indication of discrimination of the different stimuli
because these elicit different responses (Cooper and Burghardt 1990; Cooper
1994, 1998). Usually, a higher TF rate indicates a higher “interest” for a given
stimulus, which, depending on the context, is often considered a proxy of
preference of that scent (e.g., different prey types or potential mates), or
an indication that the stimulus is novel and elicits a longer chemosensory
investigation (e.g., familiar vs. unfamiliar conspecifi c recognition). Pungency
controls, such as cologne, are often used to assess responses to odorous,
readily detectable chemicals, which are not relevant to the discrimination
being studied (Cooper 1998; Cooper et al. 2003). Differences in latencies to
the fi rst TF after presentation of the scent stimulus are also used to indicate
detection and discrimination of different stimuli.
In the case of lizards that do not usually tongue-fl ick during swab tests,
such as some iguanids, other similar quantifi able chemosensory behaviors
such as labial-licking, chin-rubbing or gular pumping are used (e.g., Wilgers
and Horne 2009). To assess the preference, or avoidance, of particular
stimuli, such as scent-marks from different individual males, many tests
measure changes in behavior (e.g., locomotory activity) or time spent by the
experimental lizard in different areas, or refuges, with substrates labeled
with different chemical stimuli (e.g., Aragón et al. 2001c; Bull et al. 2001;
Martín and López 2006a).
3.3 CHEMICAL COMPOUNDS AS POTENTIAL LIZARD
PHEROMONES
Lizards have several possible sources of chemical compounds that may
potentially function as pheromones, such as the skin and secretions by large
specialized holocrine glands (e.g., precloacal/preanal or cloacal/urodeal
glands) (Mason 1992; Labra et al. 2002). Reproductive hormones, such as
testosterone, regulate the secretory activity of these glands (Fergusson et
al. 1985; Mason 1992; Moore and Lindzey 1992), which indicates their role
in reproduction. Some studies use gas chromatography coupled with mass
spectrometry (GC-MS) for identifi cation and quantifi cation of lipophilic
compounds in secretions (Fig. 3.1). Less frequently, the proteinacious
fractions of secretions have been studied with different electrophoresis
techniques, especially in the past. Studies using both methods have
described the mixtures of chemical compounds secreted by lizard glands
in a few lizard species from limited taxonomic groups (reviewed in Weldon
et al. 2008) (Table 3.1).
46 Reproductive Biology and Phylogeny of Lizards and Tuatara
3.3.1 Skin Lipids
The skin of lizards typically contains mainly fatty acids, hydrocarbons,
alcohols, steroids and waxy esters, among others. The characteristic
combinations of chemicals and their concentrations vary among species
(Roberts and Lillywhite 1980; Weldon and Bagnall 1987; Mason and Gutzke
1990). The main role of these lipids is to protect the skin against water loss
(Roberts and Lillywhite 1980). Nevertheless, skin compounds might also
be used as pheromones, at least for species, and sex recognition in short-
distance interactions in many species. For example, female leopard geckos,
Eublepharis macularius (Eublepharidae), elicit courtship behavior from males,
but when females are shedding the skin, males respond aggressively, as
if females were competitor males. These responses may be explained by
the presence in the skin of females, but not in the skin of males, of long-
Fig. 3.1 A typical chromatogram of the lipophilic fraction of femoral gland secretions of the
ocellated lizard, Timon lepidus (Lacertidae). The identi cation of the peaks of the major
compounds are indicated: 1: Hexadecanoic acid; 2: Octadecenoic acid; 3: Octadecanoic
acid; 4: I-Tocopherol; 5: Cholesterol; 6: α-Tocopherol; 7: Cholestan-3-one; 8: Campesterol;
9: Ergostanol; 10: Sitosterol; 11: Ergostanol, methyl derivative. Original.
Pheromones and Chemical Communication in Lizards 47
Table 3.1 Lizard and tuatara species for which lipophilic chemical compounds, with the potential
of being pheromones, found in gland secretions (mostly femoral or preclocal/preanal glands
except those indicated) have been described.
INFRAORDER, Family Species Author
IGUANIA:
Agamidae Acanthocercus atricollis Martín et al. 2013c
Uromastyx aegyptia Martín et al. 2012
Uromastyx hardwickii Chauham 1986
Crotaphytidae Crotaphytus bicinctores Martín et al. 2013b
Iguanidae Iguana iguana Weldon et al. 1990; Alberts
et al. 1992a,b
Dipsosaurus dorsalis Alberts 1990
Liolaemidae Liolaemus spp.
(20 species)
Escobar et al. 2001
Liolaemus fabiani Escobar et al. 2003
GEKKOTA:
Gekkonidae Cyrtopodion scabrum Khannoon 2012
Hemidactylus aviviridis Chauham 1986; Khannoon
2012
Hemidactylus turcicus Khannoon 2012
SCINCOMORPHA:
Cordylidae Cordylus giganteus
(femoral and generational glands)
Louw et al. 2007, 2011
Teiidae Tupinambis merianae Martín et al. 2011
Lacertidae Acanthodactylus boskianus Khannoon et al. 2011a,
2013
Acanthodactylus erythrurus López and Martín 2005d
Iberolacerta cyreni (=Lacerta
monticola cyreni)
López and Martín 2005c;
López et al. 2006
Iberolacerta monticola (=Lacerta
monticola monticola)
Martín et al. 2007c; López
et al. 2009a
Lacerta schreiberi López and Martín 2006
Lacerta viridis Kopena et al. 2009
Podarcis gaigeae Runemark et al. 2011
Podarcis hispanica
(species complex)
Martín and López 2006c;
Gabirot et al. 2010a,
2012a,b
Podarcis lilfordi Martín et al. 2013a
Podarcis muralis Martín and López 2006c;
Martín et al. 2008
Psammodromus algirus Martín and López 2006d
Psammodromus hispanicus López and Martín 2009a
Timon lepidus (=Lacerta lepida) Martín and López 2010a
Zootoca vivipara (=Lacerta
vivipara)
Gabirot et al. 2008
Table 3.1 contd....
48 Reproductive Biology and Phylogeny of Lizards and Tuatara
chain methyl ketones, which are lost after shedding the skin (Mason and
Gutzke 1990). Interestingly, similar methyl ketones are found in the skin
of female garter snakes, Thamnophis sirtalis parietalis, where they serve as
sex attractiveness pheromones (Mason et al. 1990).
3.3.2 Compounds in Femoral and Precloacal Gland Secretions
Many lizards have femoral or precloacal/preanal glands, which are
probably homologous with each other, differing only in their position in
different species (Gabe and Saint Girons 1965). These are holocrine glands
that produce an abundant secretion that is slowly secreted through the
epidermal pores of femoral, preanal or precloacal glands (Cole 1966; Alberts
1993). Secretion is usually more abundant in males (i.e., it is often absent in
females although they have vestigial pores) and during the mating season
(Alberts et al. 1992b; Martins et al. 2006), and production is stimulated by
androgenic hormones (e.g., Fergusson et al. 1985). Owing to the ventral
location of femoral and precloacal pores, secretions are passively deposited
on substrates as lizards move, which may serve to scent mark territories
(see Section 3.5.1). Moreover, active rubbing of the pores against substrate
has been observed.
Both lipophilic and proteinaceous compounds are generally found in
femoral (or precloacal) secretions. Proteins may be the major component in
secretions. Although they show characteristic and stable species-dependent
patterns, minor differences among them among individuals might be used
in individual recognition (Alberts 1990, 1991; Alberts and Werner 1993;
Alberts et al. 1993).
In addition to these roles of proteinaceous compounds, lipophilic
compounds may be important for communication in a reproductive context
(e.g., Martín and López 2006a). Lipids have the advantage of being more
INFRAORDER, Family Species Author
Scincidae Plestiodon laticeps (=Eumeces
laticeps)
(urodeal gland)
Cooper and Garstka 1987
Egernia striolata
(feces)
Bull et al. 1999a
Amphisbaenidae Blanus cinereus López and Martín 2005b,
2009b
RHYNCHOCEPHALIA Sphenodon punctatus
(cloacal gland)
Flachsbarth et al. 2009
Table 3.1 contd.
Pheromones and Chemical Communication in Lizards 49
volatile and have a high degree of molecular diversity, which increases the
potential information content of a pheromone. In addition, the production
of lipids is regulated by the general metabolism, and, thus, secreted lipids
could be directly related to, and thereby potentially signal, the characteristics
and condition of the signaler. Typical lipophilic compounds in femoral
or precloacal gland secretions of lizards are steroids and carboxylic or
fatty acids, as major compounds, together with usually minor amounts
of alcohols, carboxylic acid esters (=waxy esters), squalene, tocopherol,
ketones, aldehydes, furanones, alkanes or amides, and other minor and
less frequent compounds (reviewed in Weldon et al. 2008).
3.3.2.1 Steroids
Among the lipids found in gland secretions of lizards, steroids are usually
the most abundant, with cholesterol being in many cases the main
compound (Weldon et al. 2008). However, it is likely that cholesterol or
other steroids are, at least initially, only useful to form an unreactive apolar
“matrix” that holds and protects other lipids in the scent marks (Escobar et
al. 2003). Nevertheless, the relative amount of cholesterol, for example, may
depend on body size in male rock lizards, Iberolacerta cyreni (Lacertidae),
suggesting a signaling function in male intrasexual relationships (Martín
and López 2007; see Section 3.5.2).
Every lizard species seems to have a specifi c combination of steroids that
appear in roughly similar relative proportions in secretions of all individuals,
although there is interindividual variability in the exact proportions of each
steroid. Cholesterol is the most abundant steroid in many species but not
in others. For example, in green lizards, the main steroids are ergostanol
and cholestanol in the Schreiber’s green lizard, Lacerta schreiberi (López and
Martín 2006) and cholestanol and cholesterol in the ocellated lizard, Timon
lepidus (Martín and López 2010a) (Fig. 3.1). Campesterol is the main steroid
in Psammodromus spp. (Lacertidae) lizards (Martín and López 2006d; López
and Martín 2009a). In the green iguana, Iguana iguana (Iguanidae), lanosterol
is the most abundant steroid, followed by campesterol and cholesterol
(Weldon et al. 1990; Alberts et al. 1992a). In Liolaemus spp. (Lioalemidae)
lizards, cholesterol and cholestanol are the main steroids (Escobar et al. 2001).
In the Great Basin collared lizard, Crotaphytus bicinctores (Crotaphytidae), in
addition to the ubiquitous cholesterol, two triunsaturated steroids, cholesta-
2,4,6-triene and cholesta-4,6,8(14)-triene, are the other two main steroids in
secretions (Martín et al. 2013b). Other steroids, such as cholesta-3,5-diene,
stigmasterol, cholestan-3-one and sitosterol are also commonly found in
secretions of many lizards in lower proportions (Weldon et al. 2008), together
with a large variety of derivatives and unidentifi ed (probably unknown)
50 Reproductive Biology and Phylogeny of Lizards and Tuatara
steroids. Many of these steroids are of vegetal (=phytosterols) or microbial
origin that have to be obtained from the diet, suggesting a relationship
between diet and characteristics of gland secretions. Interestingly, some
lizards secrete steroids that are precursors of vitamins, such as cholesta-5,7-
dien-3-ol (=dihydrocholesterol; a precursor of vitamin D3) and ergosterol
(provitamin D2). Thus, diet quality may affect quality of pheromones,
which may explain why, in some lacertid lizards, females prefer the scent
of males with high proportions of these provitamins (López and Martín
2005a; Martín and López 2006a,b; see Sections 8.5.3 and 8.6).
3.3.2.2 Fatty acids
Fatty or carboxylic acids, both saturated and unsaturated, are abundant
in most glandular secretions of lizards. Hexadecanoic (=palmitic) and
octadecenoic (=oleic) acids are present in most lizard species. Other fatty
acids commonly found, although in lower proportions, are tetradecanoic
(=myristic), hexadecenoic (=palmitoleic), octadecanoic (=stearic),
9,12-octadecadíenoic (=linoleic), and eicosanoic (=arachidic) acids, among
others (Weldon et al. 2008).
The fatty acids are typically found in series, which vary with respect to
the number of carbons that form the hydrocarbon chain and vary among
species. This has been interpreted as an adaptation to maximize effi cacy of
substrate scent marks under different microclimatic conditions, with fatty
acids of high molecular weight, and, therefore, less volatile, being favored
in areas with higher temperatures or greater humidity (Alberts 1992).
Fatty acids can also appear in the form of ethyl esters, which confer more
stability. For example, in Iguana iguana, from warm wet tropics, the chain
lengths of fatty acids found in femoral secretions range between C14 and C26
(Weldon et al. 1990; Alberts et al. 1992a), while Iberolacerta cyreni lizards from
cold, dry high mountains, have fatty acids between C6 and C22 (López and
Martín 2005c). Also, in the Iberian wall lizard, Podarcis hispanica (Lacertidae),
populations from relatively dryer habitats with mild temperatures have
a higher proportion of fatty acids of low molecular weight (Martín and
López 2006c). However, under the warmest and driest conditions, where
evaporation rates are higher, P. hispanica also have the more stable ethyl
ester forms of fatty acids (Gabirot et al. 2012a). In contrast, in the Spanish
sand lizard, Psammodromus hispanicus (Lacertidae), which inhabits grassy
substrates where scent marks could be useless, there is a great abundance
of fatty acids with a low number of carbons, especially dodecanoic acid,
which is highly volatile and, thus, might be more suitable for short-distance
communication not requiring a durable signal (López and Martín 2009a).
Pheromones and Chemical Communication in Lizards 51
Within the same species and population, dietary or hormonal differences
among individuals might result in different proportions of fatty acids. For
example, in Iberolacerta cyreni, proportions of oleic acid in femoral secretions
of males were positively related to body condition of males, suggesting that
the amount of oleic acid secreted may refl ect the amount of body fat reserves
of a male (Martín and López 2010b). Also, in Iguana iguana, the proportion of
unsaturated fatty acids increases during the mating season when androgens
also increase, which may enhance volatility and detectability of secretions
(Alberts et al. 1992a,b). Stressful situations, such as increased predation
risk levels, may also alter proportion of lipids (fatty acids and steroids) in
secretions, probably due to the increase in circulating levels of corticosterone
and its effect on lipid metabolism (Aragón et al. 2008).
In secretions of the Argentine black and white tegu lizard, Tupinambis
merianae (Teiidae), there are large (>25%) amounts of 9,12-octadecadienoic
acid (= linoleic acid) (Martín et al. 2011). This unsaturated fatty acid has
been found in secretions of other lizards but always in very small amounts
(Weldon et al. 2008). Secretion of large amounts of linoleic acid must be
costly for lizards because it is one of two essential polyunsaturated fatty
acids that many animals must ingest for good health. Given the dietary
origin and the important functions of linoleic acid in metabolism, its
actual function in femoral secretions of T. merianae must be suffi ciently
important to divert it from metabolism and “secrete” it from the body. It is
likely that only males able to get an adequate dietary supply could secrete
it. Therefore, the presence of linoleic acid in secretions might signal male
quality (see Section 8.6).
3.3.2.2 Alcohols
Glandular secretions of lacertid lizards usually also include some alcohols,
but alcohols were absent in secretions of several iguanid species (although
relatively few iguanids have been studied). Lacertid lizards usually have
alcohols such as hexadecanol or octadecanol in low proportions, but this
does not imply that alcohols are unimportant. For example, in rock lizards,
Iberolacerta monticola (Lacertidae), males with femoral secretions with
higher abundances of hexadecanol and octadecanol had higher dominance
status, and males respond aggressively to these alcohols (Martín et al.
2007c; see Section 3.5.2). In spiny-footed lizards (Acanthodactylus erythrurus
and A. boskianus) (Lacertidae), long-chain alcohols (e.g., hexacosanol and
tetracosanol) are the most abundant compounds (hexacosanol is also
known as ‘ceryl alcohol’, and tetracosanol as ‘lignoceric alcohol’). These
alcohols may form waxy esters that make femoral secretions more cohesive,
52 Reproductive Biology and Phylogeny of Lizards and Tuatara
enhancing durability of pheromonal signals in the dry habitat of these
lizards (López and Martín 2005d; Khannoon et al. 2011a). These alcohols
may also be involved in signaling dominance, as suggested by the avoidance
or aggressive behaviors of male A. boskianus lizards in response to these
compounds (Khannoon et al. 2011b).
3.3.2.3 Other lipophilic compounds
Femoral or precloacal secretions of lizards also contain other types of
compounds, usually as minor components, but in some cases as major
compounds. Even if they are not especially abundant, such compounds are
potentially important in communication, either directly or by enhancing
the signaling function of other compounds.
Esters of a long chain fatty acid and a long chain fatty alcohol (= waxy
esters) are found in secretions of many lizards (Weldon et al. 2008). Usually,
there are diverse esters of the fatty acids hexadecanoic (=hexadecanoates),
octadecenoic (=octadecenoates) and octadecanoic acids (=octadecanoates),
linked to alcohols such as tetradecanol, hexadecanol or octadecanol. These
are waxy compounds that may confer a greater stability to secretions,
allowing scent marks to persist longer in very dry and warm or very
humid environments. For example, waxy esters of fatty acids are especially
abundant in femoral secretions of Crotaphytus bicinctores, in which the
high proportion of waxy esters derived from the long chain eicosanoic
(=icosanoates) and docosanoic acids (=docosanoates) is noteworthy (Martín
et al. 2013b). This abundance of more stable waxy esters may protect scent
marks from rapid evaporation in the xeric warm conditions in the habitat
of this lizard.
Squalene is a hydrocarbon and a triterpene, and is a natural and vital
part of the synthesis of all plant and animal sterols, including cholesterol,
steroid hormones, and vitamin D. It is a common constituent in secretions
of many lizards (Weldon et al. 2008), in which it might have a role as an
antioxidant. For example, in the common lizard, Zootoca vivipara (Lacertidae)
(formerly Lacerta vivipara), the lipids in femoral secretions would oxidize
very quickly under the humid conditions of its environments (e.g., wet
meadows, swamps, damp forests, etc.), but squalene might stabilize
the other lipid fractions by limiting oxidation (Gabirot et al. 2008).
Chemosensory discrimination of sex in the fossorial amphisbaenian Blanus
cinereus, which shows precloacal gland secretions in both sexes, may be
based on the much greater proportions of squalene found in secretions of
males (López and Martín 2005b). The detection of squalene that ‘‘signals’’
male identity elicits, only in males, aggressive responses similar to those
Pheromones and Chemical Communication in Lizards 53
observed in agonistic interactions between males in a reproductive context
(López and Martín 2009b).
Tocopherol (=vitamin E) is the main compound in femoral secretions
of green lizards (Lacerta schreiberi, L. viridis and Timon lepidus) (López and
Martín 2006; Kopena et al. 2009; Martín and López 2010a) (Fig. 3.1), but it
is also found in other lizards in much lower amounts (López and Martín
2005d; Martín and López 2006c; Gabirot et al. 2008). Tocopherol is a typical
antioxidant that may protect other compounds in the secretions from
oxidation in wet environments, but it may also have a signaling function
in female mate choice (Kopena et al. 2011; see Section 3.6).
Ketones can also appear in minor proportions in secretions of some
lizards. They might have an important role, as yet untested, in communication
in some cases. For example, the presence of a series of C17–C25 saturated
methyl ketones with mostly odd-numbered carbon chains is noteworthy
in preanal gland secretions of male blue-headed tree agamas, Acanthocercus
atricollis (Agamidae) (Martín et al. 2013c). A similar bishomologous series
of methyl ketones were found, apparently homplasically, in the femoral
gland secretions of the phylogenetically distantly unrelated South African
giant girdled lizard, or sungazer, Cordylus giganteus (Cordylidae) (Louw et
al. 2007) and in the skins of females geckos Eublepharis macularius (Mason
and Gutzke 1990) and female Thamnophis sirtalis snakes (Mason et al. 1990).
In the latter, they have a prominent role in the social and sexual behavior.
Aldehydes, such as tetradecanal or hexadecanal, are also often found
in secretions (Weldon et al. 2008). These are highly odoriferous compounds
that might facilitate detection by conspecifi cs of femoral secretions after
they are deposited. Aldehydes have been found in some lizard species,
but not in other phylogenetically related species (e.g., they are abundant in
Psammodromus algirus but do not appear in P. hispanicus; Martín and López
2006d; López and Martín 2009a). This difference suggests the hypotheses
that presence of aldehydes in secretions might depend on the environment
or social behavior of each species.
Other minor compounds such as amides (e.g., octadecenamide) and
furanones (= lactones of fatty acids) have also been found in many lizards
(Weldon et al. 2008). Furanone derivatives are frequently found in nature
as pheromones, fl avor compounds or secondary metabolites, but their
potential function in lizard secretions is unknown. Also, a large number of
homologous long-chain alkanes were identifi ed in the precloacal secretions
of 20 Liolaemus lizard species (Escobar et al. 2001, 2003). However, alkanes
have not been found in the secretions of other lizards. In Liolaemus the
alkanes might have come from the skin surrounding the precloacal pores
rather than from the precloacal secretion per se. Nevertheless, pentacosane
was found in femoral secretion of Cordylus giganteus (Louw et al. 2007)
54 Reproductive Biology and Phylogeny of Lizards and Tuatara
and octacosane, nonacosane, triacontane and hentriacontane in the
Balearic lizard, Podarcis lilfordi (Lacertidae) (Martín et al. 2013a). Finally,
monoglycerides of fatty acids and glycerol monoethers of long chain
alcohols were identifi ed in femoral secretions of Acanthodactylus boskianus
(Khannoon et al. 2011a). These compounds have been rarely identifi ed
from nature, have not yet been found in other lizards, and their possible
signaling function is unknown.
3.3.3 Compounds in Generation Glands
In addition to femoral glands, cordylid lizards have generation glands
as a potential source of pheromones (Van Wyk and Mouton 1992). These
glands are formed by holocrine secretory cells located in the beta-layer of
the epidermis, and may occur in different body locations, such as in the
femoral, precloacal, antebrachial (forearm), and dorsal epidermal regions.
A chemical analysis of the secretion of generation glands of Cordylus
giganteus, identifi ed alkenes, carboxylic acids, alcohols, ketones, aldehydes,
esters, amides, nitriles and steroids (Louw et al. 2011). The most abundant
compound was hexadecanoic acid. Interestingly, there are important
differences with compounds identifi ed in the femoral gland secretions of
this lizard species (Louw et al. 2007). Cholesterol, a major component in
femoral secretions does not occur in the generation glands, while alkanes do
not occur in femoral secretions. These differences were explaining because
generation glands are glandular scales, forming part of the lizard’s skin.
3.3.4 Compounds in Cloacal Secretions and Feces
Little is known about the functions of the several glands (urodeal,
proctodeal, etc.) found in the cloacas of lizards. The initial function of these
glands seems to be provision of lubrication to the intestinal tract to facilitate
expulsion of excrements or to facilitate mating. However, the glands also
are a potential source of pheromones (Trauth et al. 1987; Cooper and Trauth
1992). This is a relatively little explored topic, but cloacal secretions might
be of great importance in chemosensory communication, especially in the
groups of lizards lacking femoral or precloacal glands (e.g., Gonzalo et al.
2004), or in species in which females have vestigial pores with little secretion.
For example, in the broad-headed skink, Plestiodon laticeps (Scincidae), the
dorsal cloacal glands may produce a species-identifying pheromone present
in both sexes that may be useful to discriminate among conspecifi c male
sexual competitors in P. laticeps and closely related skinks (Cooper et al.
1986; Cooper and Vitt 1987; Trauth et al. 1987). The urodaeal gland of female
Pheromones and Chemical Communication in Lizards 55
black-lined plated lizards, Gerrhosaurus nigrolineatus (Gerrhosauridae) was
hypothesized to be a source of female sex pheromone, while the dorsal
and ventral glands may be the source of species-identifi cation or male
pheromones (Cooper and Trauth 1992). The precise chemical identity of
such pheromones is unknown. However, when different chemical fractions
of the whole urodaeal glands of female P. laticeps were presented to males,
their tongue-fl ick rates were higher in response to neutral lipids than to
other fractions (Cooper and Garstka 1987). Pheromonal activity appears
to reside in the neutral lipid fraction, which includes steryl and wax esters
and mono-, di- and triacylglycerols, but not in acidic or basic lipids, or in
carbohydrate or the protein fractions.
The cloacal gland secretion of the tuatara, Sphenodon punctatus, contains
a glycoprotein and a complex mixture of triacylglycerols derived from
unusual medium chain-length fatty acids as major constituents (Flachsbarth
et al. 2009). However, it is not clear that these compounds can function as
pheromones, because tongue-fl icking is not observed in social interactions
of tuataras (Gans et al. 1984), although it remains possible that olfaction
might be used because tuataras respond by biting to swabs impregnated
with prey chemicals (Cooper et al. 2001).
In several lizard species, chemicals with pheromonal function, probably
coming from the cloacal glands, may be secreted onto the surface of the feces
or scats as these are deposited by the lizard. Compounds in feces seem to
be useful for scent-marking and conspecifi c recognition. Scent from feces
may provide information on familiarity, relatedness, or body size of the
producer (Duvall et al. 1987; Carpenter and Duvall 1995; López et al. 1998;
Bull et al. 1999a,b, 2001; Aragón et al. 2000; Moreira et al. 2008; Wilgers and
Horne 2009). Compounds from feces with properties of pheromones have
not been identifi ed, but they are probably a combination of several lipids
as, in the tree skink, Egernia striolata (Scincidae), they are contained in scat
extracts made with organic solvents (dichloromethane); fractionation of
the scats with different solvents (pentane and methanol) led to loss of the
unique signals needed for individual recognition (Bull et al. 1999a).
Finally, compounds from cloacal glandular secretions that have
pheromonal activity may be added to copulatory plugs of males. Male
Iberolacerta monticola, can distinguish their own copulatory plugs from those
of other males and can even assess the dominance status of other males
by chemosensory cues from copulatory plugs (Moreira et al. 2006). This
suggests that copulatory plugs may allow males to “scent-mark” the female
body during copulations and that this behavior may infl uence mating
decisions of other males under selective pressures of sperm competition
(e.g., a male might recognize and avoid displacing its own plugs while
displacing plugs of other males). These hypotheses remain to be tested.
56 Reproductive Biology and Phylogeny of Lizards and Tuatara
3.4 CHEMOSENSORY RESPONSES TO SPECIFIC CHEMICAL
COMPOUNDS
Many studies have shown the ability of lizards to use their chemosensory
systems to discriminate the scent of conspecifics from scents of
heterospecifi cs (e.g., Cooper and Vitt 1987; Barbosa et al. 2006; Gabirot et al.
2010a,b), scents of males from females (e.g., Cooper and Trauth 1992; Cooper
and Steele 1997; Cooper et al. 1996; Labra and Niemeyer 1999; López and
Martín 2001a; Khannoon et al. 2010) and reproductive condition of females
(e.g., Cooper and Vitt 1984; Cooper and Pérez-Mellado 2002). Lizards also
use pheromones to discriminate the scent of familiar from unfamiliar
individuals and self-recognition (e.g., Alberts and Werner 1993; Cooper et
al. 1999; Aragón et al. 2001a,b; Carazo et al. 2008).
Only a few studies have examined whether lizards can discriminate
between the different types of chemical compounds found in these scents.
Some studies used the TF rates of lizards to scent stimuli presented on cotton
swabs to examine, within a foraging context, discrimination of compounds
found in the insect prey of lizards (Cooper and Pérez-Mellado 2001; Cooper
et al. 2002a,b). Podarcis lilfordi, can discriminate between lipids, proteins, and
carbohydrates (Cooper et al. 2002a) and also among different lipids, such as
glycerol, cholesterol, and oleic and hexadecanoic acids (Cooper et al. 2002b).
With respect to compounds found in glandular secretions of lizards,
another study measured the TF responses of female Podarcis hispanica
to two lipids (cholesterol and cholesta-5,7-dien-3-ol) (Martín and López
2006e). These steroids are major compounds in femoral secretion of males
(Martín and López 2006c). Females discriminate between these two lipids,
showing higher TF responses to cholesta-5,7-dien-3-ol and are able to
assess differences in its concentration, responding more strongly to higher
concentrations. These results, together with the female preference for areas
scent marked by males with higher proportions of this steroid in their
secretions (López and Martín 2005a; see Section 3.5.3), suggest that cholesta-
5,7-dien-3-ol is a ‘‘key’’ pheromonal compound for this lizard.
In some cases, intersexual differences in chemosensory responses
suggest that different compounds may carry different messages for males
and females. Thus, female Iberolacerta cyreni have higher TF responses to
cholesta-5,7-dien-3-ol and to ergosterol than to cholesterol, whereas the
opposite is found in males (Martín and López 2006a, 2008a). This is probably
explained by the preference of females for scent marks of males with higher
proportions of cholesta-5,7-dien-3-ol and ergosterol in their secretions,
which is related to the “quality” of those males (Martín and López 2006a,b;
see Section 3.5.3), whereas in males cholesterol might signal the body size
of a potential male opponent (Martín and López 2007; see Section 3.5.2).
Pheromones and Chemical Communication in Lizards 57
Female Iberolacerta cyreni can also discriminate among fatty acids found
in femoral gland secretions of males, such as oleic acid and hexadecanoic
acid, and can assess differences in their concentration (Martín and López
2010b). This discrimination might be important for females because
the amount of fatty acids secreted may refl ect the amount of body fat
reserves of a male. The presence of both saturated (e.g., hexadecanoic) and
unsaturated (oleic) fatty acids in the males’ secretions might allow the scent
signal to function in varying environmental conditions because at ambient
temperatures, unsaturated fatty acids may be accessible as liquids, whereas
saturated fatty acids may be waxes. Thus, it is likely that females actually
responded to the whole mix of fatty acids usually found in males’ secretion
or that under different temperature conditions, some fatty acids were more
effective than others in eliciting chemosensory exploration of females.
Alcohols can also be detected by lizards. Male and female Podarcis
hispanica lizards can discriminate among alcohols found in secretions
of males and vary tongue-fl ick rates with their concentrations (Gabirot
et al. 2012c). Male Iberolacerta monticola discriminate between different
concentrations of hexadecanol, a major compound in glandular secretions of
males, from other chemicals (Martín et al. 2007c). Moreover, males respond
aggressively to hexadecanol, but respond neutrally to other compounds
(Martín et al. 2007c). These results, together with the relationship observed
between femoral secretions with higher proportions of hexadecanol and
dominance, suggest that hexadecanol may be a reliable status badge in this
lizard. Similarly, male Acanthodactylus boskianus show avoidance behavior
for substrates marked with cholesterol and long chain alcohol blends (both
found in males’ secretions), and agonistic behavior towards these stimuli,
whereas females do not respond to these chemicals (Khannoon et al. 2011b).
The observed chemosensory responses to glandular secretions may
be a consequence of response to the combined multiple effects of different
compounds. For example, female Iberolacerta cyreni discriminate between
different concentrations of ergosterol and oleic acid presented alone and
exhibit the highest chemosensory exploration to high concentrations of
ergosterol, whereas high concentrations of oleic acid elicit tongue-fl ick (TF)
rates of a magnitude similar to those to low concentrations of ergosterol
(López and Martín 2012) (Fig. 3.2). Moreover, the highest TF rates are
directed to a mixture containing high concentrations of both compounds
combined, and there is an upper-shift of the top of the dose-response curve
by the combination of the two compounds, suggesting that there are additive
or synergic effects of these two compounds (Fig. 3.2).
58 Reproductive Biology and Phylogeny of Lizards and Tuatara
3.5 REPRODUCTIVE CHEMICAL ECOLOGY IN LIZARDS
Multiple lines of evidences show that lizards have highly developed
chemosensory abilities, including strong responses to scent of conspecifi cs,
and that most lizards produce, especially during the mating season,
glandular secretions that contain many chemicals that potentially function
as signals. Nevertheless, as noted in Section 3.1, the reproductive behavior
of lizards was long considered to be mainly based on more conspicuous
visual signals. Thus, most research on reproductive behavior of lizards
focused on colorful traits or movement displays. Relatively few studies
have considered the potential role of chemical signals in reproductive
ecology of lizards. These will be summarized in the following sections. We
are just starting to understand not only the function of specifi c chemicals in
modulating different behaviors related to reproduction and sexual selection,
but also the mechanisms that explain the use and evolutionary persistence
of these signals.
3.5.1 Scent-Marking
Glandular secretions, feces, or urine are very often used for scent-marking
of substrates by many terrestrial vertebrates, including many lizards. These
scent-marks identify territorial boundaries or attract mates (reviewed in
Müller-Schwarze 2006; Mason and Parker 2010). Scent-marking a territory
Fig. 3.2 Dose-dependent and additive effects of two compounds from males’ femoral gland
secretions on chemosensory responses of female Iberian rock lizards (Iberolacerta cyreni).
Number (mean + SE) of tongue- icks directed to swabs by female lizards in response to
cotton-tipped applicators bearing different concentrations (0, 5, 20, or 40 mg/mL) of oleic acid
(Ole) and ergosterol (Erg) (standard compounds) presented alone or together, all dissolved in
DCM. From López, P. and Martín, J. 2012. Chemical Senses 37: 47–54. Figure 3.
Pheromones and Chemical Communication in Lizards 59
can be a simple and effective method to inform conspecifi cs about the
identity and characteristics of the male that defends the marked territory.
Many lizards can scent-mark their territories by using femoral or cloacal
secretions and/or feces. Semiochemicals in these scent marks are known
to convey information on sex, age, body size, dominance status or health
condition of the signaler (reviewed in Mason 1992; Mason and Parker 2010;
Martín and López 2011).
If the information in the scent-mark is reliable (e.g., Martín and López
2006b; Kopena et al. 2011), the signaler will benefi t from this advertisement,
for example, by repelling rivals or attracting mates (Martín and López
2012). Receivers of the signal may gain benefi ts by using information
about territorial status and dominance obtained from scent-marks into
their decisions about aggressive behavior toward the scent-marking male
(e.g., Carazo et al. 2007, 2008; López and Martín 2011) or about mate choice
(e.g., Martín and López 2000, 2006a,b; López et al. 2002b; López and Martín
2005a; Olsson et al. 2003). Scent-marks may be important in reproductive
behavior and sexual selection of many lizards (see below).
3.5.2 Intrasexual Relationships Between Males and Social
Organization
When competing for access to mates, males use cues from their rivals to
judge relative fi ghting ability and to evaluate their chances of success
in a potential future agonistic contest. In lizards, chemical signals may
be a vital component of male-male contests informing males of a rival’s
quality or intentions. Pheromones may signal a male’s dominance status,
or characteristics related to fi ghting ability or dominance such as body
size, through rates of production and/or the quality of the glandular
secretions (Alberts et al. 1992b; López et al. 2003b; Martins et al. 2006; Martín
et al. 2007c). In many cases, pheromones also allow lizards to discriminate
between familiar and unfamiliar males and may allow true individual
recognition (i.e., based on individual identity cues) (Aragón et al. 2001a;
Carazo et al. 2008).
Pheromones affect intrasexual relationships in male lizards in two
ways. First, pheromones deposited in substrate scent-marks can provide
information in absence of the signaler on the presence of previously known
individual rivals or on the fi ghting potential of unfamiliar individuals
(Aragón et al. 2000, 2001a; Labra 2006; Carazo et al. 2007, 2008). This
information may affect behavior and space-use by other males that sample
the scent-marks (Alberts et al. 1994; Aragón et al. 2001c, 2003; Labra 2006).
Second, pheromones may be used during actual agonistic encounters,
for example, to recognize rival males (Cooper and Vitt 1987; López et al.
60 Reproductive Biology and Phylogeny of Lizards and Tuatara
2002a). This is important because when two males interact, they become
familiar and establish their relative dominance relationship, which allows
them to decrease the aggressiveness in successive encounters (López and
Martín 2001b). In some cases, recognition of familiar lizards or of specifi c
individuals may be predominantly based on pheromonal cues. In Podarcis
hispanica, resident males respond more aggressively towards unknown or
familiar males experimentally impregnated with scents from unfamiliar
males than to familiar males or unknown males impregnated with scents
of familiar males (López and Martín 2002) (Fig. 3.3).
Chemical rival recognition may be used in other situations. Males of
many species of lizards show conspicuous breeding colors, but, in some
species, young or competitively inferior males conceal their sexual identity
by mimicking a female-like dull coloration that allows them to evade
aggression from dominant males and to adopt an alternative satellite-
sneaking mating tactic. In two experiments, scent and coloration of satellite
males, were manipulated in Psammodromus algirus (López et al. 2003b) and
Augrabies fl at lizards, Platysaurus broadley (Cordylidae) (Whiting et al. 2009).
In both species, deceptive coloration was effective in avoiding aggression
at long distance. However, at close range, dominant males used chemical
signals to identify satellite males, as shown by aggressive responses toward
satellites.
Fig. 3.3 Role of pheromones in intrasexual agonistic behavior of Iberian wall lizards (Podarcis
hispanica). Number (mean + SE) of aggressive (black bars) and neutral (white bars) interactions
in the rst contest of a resident male with an intruder male impregnated with his own odor, in
posterior contests with the same familiar male bearing his own odor (Fm/Fo) or impregnated
with odor of an unfamiliar male (Fm/Uo), and in posterior contests with unfamiliar males
impregnated with odor of a familiar male (Um/Fo) or bearing their own odor (Um/Uo). From
López, P. and Martín, J. 2002. Behavioral Ecology and Sociobiology 51: 461–465. Figure 1.
Pheromones and Chemical Communication in Lizards 61
In some animals, males may identify territory owners by directly
comparing the scent of substrate marks with the scent of any conspecifi c they
encounter nearby, i.e., by scent-matching (Gosling and McKay 1990). This
may also occur in lizards. Thus, when an intruding male Iberolacerta cyreni,
explores substrate scent marks, if he subsequently fi nds a rival male whose
scent experimentally matches that of scent marks (considered presumably
to be the territory owner), the intruding male delays time until the fi rst
agonistic interaction, reduces the intensity and number of fi ghts, and wins
fewer interactions than when encountering other non-matching individual
males (López and Martín 2011). Therefore, males may use scent-matching
as a mechanism to assess the ownership status of other males, which could
contribute to modulation of further intrasexual aggressions.
However, the chemical basis of the assessment of rival dominance
status or fi ghting ability is poorly known. Assessment might be affected by
changes in concentrations of some chemicals in scents that are correlated
with traits that affect fi ghting ability. In Iguana iguana, femoral glandular
productivity, pore size, and the percentage of lipids in the secretions are
correlated with plasma testosterone levels in dominant, although not in
subordinate, adult males (Alberts et al. 1992b). Proportions of cholesterol in
femoral secretions of male Iberolacerta cyreni increase with body size (López
et al. 2006). These males discriminate chemically and respond aggressively
to cholesterol stimuli presented on swabs (Martín and López 2008a), and,
moreover, when cholesterol in the body scent of males is experimentally
increased, they win more frequently agonistic interactions (Martín and
López 2007), suggesting that high concentration of cholesterol may signal
greater fi ghting ability linked to larger body size. This may be a reliable
signal, if higher proportions of cholesterol in secretions indicate higher sex
steroid (i.e., testosterone) levels that also determine aggressiveness levels
(Alberts et al. 1992b; Sheridan 1994).
Similarly, in male Iberolacerta monticola, dominant males produce
femoral secretions with higher proportions of two alcohols (hexadecanol
and octadecanol) (Martín et al. 2007c) (Fig. 3.4). Males discriminate different
concentrations of hexadecanol from other chemicals found in secretions
and respond aggressively towards hexadecanol according to their own
dominance status, but respond neutrally to other chemicals. The signal may
be reliable because, given that hexadecanol elicits aggressive responses of
other males, only truly dominant males with a high fi ghting potential should
chemically signal their status. Also, it might be physiologically costly to
produce femoral secretions with high amounts of hexadecanol. Consistent
with this view, dominant males are healthier (i.e., have a stronger immune
response), which might allow them to afford secreting greater quantities
of compounds that signal a high dominance status (Martín et al. 2007c).
62 Reproductive Biology and Phylogeny of Lizards and Tuatara
3.5.3 Female Mate Choice
Some fi eld studies suggest that females of some lizard species do not
choose males, but base their space-use on the quality of a territory (e.g.,
thermal characteristics, abundance of food or refuges, etc.) rather than on
the quality of the male that defends that territory (e.g., Hews 1993; Calsbeek
and Sinervo 2002). Males would only defend these favorable territories from
other males to increase their access to females. However, it is still possible
that females might be attracted to a territory by male signals that may be
used as ‘‘public information’’ to assess the quality of a territory, or through
being ‘‘lured’’ by male signals that resemble food. In this context, in lizard
species in which males scent-mark territories, pheromones may have an
important role in female space-use and mate choice.
Other studies suggest that female lizards of some species might use
some chemical compounds in the scent-marks of males to select areas scent
marked, and, therefore, occupied by preferred potential mates (Martín
and López 2006a, 2012, 2013a; Johansson and Jones 2007). On the other
hand, a pre-existing sensory bias for food chemicals might also explain
the chemosensory preferences of female lizards for some compounds in
the scent-marks of males (Martín and López 2008). However, irrespective
of the causes underlying decisions by females to spend more time in a
given scent marked area, this decision about use of space will increase the
probability of mating with the male that has scent marked the selected area
(Martín and López 2012). Females may try to reject mating advances from
“undesired” males, but males may obtain many forced matings. Therefore,
Fig. 3.4 Chemical basis of dominance status signallng in rock lizards (Iberolacerta monticola).
Relationships between dominance status scores of male lizards and PC scores obtained
from a principal components analysis on the relative proportions of chemical compoundss
in femoral gland secretions. From Martín, J., Moreira, P. L. and López, P. 2007. Functional
Ecology 21: 568–576. Figure 1.
Pheromones and Chemical Communication in Lizards 63
space-use decisions of female lizards will have direct consequences for
their reproductive success, and those space-use strategies that increase
the reproductive success of females will be evolutionary selected. As a
consequence, space-use decisions of female lizards based on scent marks
of males may have the same evolutionary consequences as “direct” mate
choice decisions of other animals.
Therefore, male lizards might use scent marks to attract females to their
territories, thus increasing the probabilities of mating with these females,
whereas females might use scent marks of males to select potential mates
or territories of high quality. But this attracting function of the scent-marks
of lizards remains little explored. One fi eld study in Iberolacerta cyreni,
showed that experimentally increasing ergosterol (a compound from
femoral secretions of males) on rock substrates inside home ranges of males
results, after some days, in increased relative densities of females, but not
of males, in those areas. This effectively results in an increase of mating
opportunities for resident males (Martín and López 2012) (Fig. 3.5). Also,
female I. cyreni prefer areas scent-marked by large/old territorial males to
those scent-marked by smaller/young satellite-sneaker males (López et
al. 2003a; Martín and López 2013a) and prefer areas scent-marked by two
territorial males to areas of similar size marked by a single territorial male
(Martín and López 2013a). The former choice might increase the probability
of obtaining multiple copulations with different males, thus favoring sperm
competition and cryptic female choice, or may be a way to avoid infertile
males (Martín and López 2013a).
To establish whether female mate choice exists, experimenters must
select appropriate criteria base on choices used by females to select a
mate (or a scent marked territory). For example, female Podarcis hispanica,
preferentially associate with areas scent-marked by males, but females do
not choose territories marked by larger versus smaller males. Taken alone,
this might suggest that mate choice by females is absent in this species
(Carazo et al. 2011). However, other experiments with this species showed
that females select scents of males with higher proportions of cholesta-5,7-
dien-3-ol (among scents from males of similar size), which are those with a
better T-cell-mediated immune response (i.e., with a better health) (López
and Martín 2005a). Similarly, female Iberolacerta cyreni, select areas scent-
marked by males with stronger immune responses, as signaled by high
ergosterol proportions in femoral secretions of males (Martín and López
2006a), or with better body condition, as signaled by high proportions of
oleic acid in secretions (Martín and López 2010b). In the same way, female
Psammodromus algirus show higher chemosensory responses to femoral
gland secretions of males with low blood parasite loads and stronger
immune responses, which is apparently signaled by higher proportions of
64 Reproductive Biology and Phylogeny of Lizards and Tuatara
Fig. 3.5 Effects of manipulation of substrate scent-marks with pheromones on density of
Iberian rock lizards (Iberolacerta cyreni). TOP: Numbers (mean + SE) of (a) adult males or
(b) adult females observed in each census of the control (black circles) and experimental
(open circles) plots before the experiment (initial) and during the four days after rocks were
supplemented with ergosterol (experimental) or a control solution. From Martín, J. and López,
P. 2012. PLoS One 7: e30108. Figure 1. BELOW: A pair of rock lizards, the territorial male (in
front) has approached to a female that was probably attracted to his area by pheromones in
scent-marks. Photograph by J. Martín.
Pheromones and Chemical Communication in Lizards 65
two alcohols (octadecanol and eicosanol) and lower proportions of their
correspondent carboxylic acids (octadecanoic and eicosanoic acids) (Martín
et al. 2007b).
Intra- and inter-sexual competition often lead to selection for different
secondary sexual traits, which may be refl ected in responsiveness to male
pheromones by female lizards. Scents of males that signal characteristics
that confer competitive advantages to males against other males (such as a
larger head or a higher bite force) are often not selected by females. Females
may respond more strongly to scents of males that signal traits of a potential
mate that are benefi cial to females, such as a higher body condition or levels
of symmetry (Iberolacerta cyreni, López et al. 2002b; Dalmatian wall lizard
Podarcis melisellensis (Lacertidae), Huyghe et al. 2012).
Similarly, in sand lizards, Lacerta agilis (Lacertidae), females do not seem
to mate selectively with larger and/or older males (Olsson and Madsen
1995), but genetic compatibility (based on the major histocompatibility
complex, MHC, dissimilarity) is the main characteristic that females select
based on the scent of a male (Olsson et al. 2003). Similar avoidance of
inbreeding based on chemical signals might function in other lizard species
(Bull and Cooper 1999; Bull et al. 2001). However, the chemical bases of this
genetic discrimination remain unknown. Selective mating with non-kin or
unrelated pairs may confer genetic benefi ts because the new combinations
of immunocompetence in offspring will defend them more effectively
against evolving parasites (Penn and Potts 1999). In contrast, female painted
dragons, Ctenophorus pictus (Agamidae), do not prefer scents from unrelated
males, which might be explained by weak selection against inbreeding in
this species (Jansson et al. 2005).
3.6 EVOLUTIONARY ORIGIN OF THE CHEMICAL SIGNALING
SYSTEM
Although some compounds “preferred” by females in the scent of males, or
that affect intrasexual relationships between males, have been identifi ed in
a few lizard species, it is not well understood why lizards can be confi dent
in the reliability and honesty of the message in the chemical signal (i.e.,
that a scent with a specifi c chemical characteristics always corresponds to
a male with some specifi c characteristics). This is, however, a prerequisite
for a signal to persist in evolutionary time.
One possible explanation may reside in the important metabolic
organismal functions of some compounds that are, however, secreted by
glands to the exterior of the body. Thus, many of the lipids commonly
found in glandular secretions of lizards, such as fatty acids and steroids,
function as signaling molecules or lipid mediators, show potent biological
66 Reproductive Biology and Phylogeny of Lizards and Tuatara
activity, and are important keys in many metabolic pathways. Some
lipophilic compounds in femoral secretions of some lizards appear to be
good candidates for conferring honesty to signals. These are α-tocopherol
(=vitamin E), cholesta-5,7-dien-3-ol (=dihydrocholesterol; a precursor
of vitamin D3), ergosterol (=pro-vitamin D2), 9,12-octadecadienoic acid
(=linoleic acid) and 5,8,11,14-eicosatetraenoic acid (=arachidonic acid). These
are essential components for metabolism that have important physiological
functions. In most cases, vertebrates can only obtain them from the diet
and their defi ciency can cause severe disorders. However, in spite of the
importance of these compounds, male lizards divert them from metabolism
to allocate them for use in femoral glandular secretions. In such cases, trade-
offs must exist between using these essential chemicals in metabolism and
using them for scent-marking. Only males that have, or are able to obtain, an
adequate supply of vitamins and essential fatty acids could allow diversion
of surplus chemicals from metabolism to social signalling. Therefore, the
presence of vitamins and essential fatty acids in relative high proportions
in secretions might honestly advertise male quality.
For example, cholesta-5,7-dien-3-ol (=pro-vitamin D3) is often found
in the skin, where it transforms into vitamin D3 after exposure to UV-B
irradiation in sunlight. Vitamin D3 is essential in calcium metabolism and
for regulation of the immune system (Fraser 1995). However, very often,
the synthesis of vitamin D3 in the skin is not suffi cient to meet physiological
requirements, and lizards require dietary intake of vitamin D (Ferguson et al.
2005). Under these conditions, vitamin D is an essential nutrient for lizards.
Therefore, when diverting pro-vitamin D from metabolism to femoral
secretions, male lizards might need to obtain more vitamin D. After
supplementation of dietary vitamin D, male Iberolacerta cyreni, increased the
proportion of pro-vitamin D3 in femoral secretions and females preferred
areas scent marked by these males with more pro-vitamin D3 (Martín and
López 2006b). This suggests that allocation of this pro-vitamin to secretions
is costly and dependent on the foraging ability of a male to obtain enough
vitamin D in the diet, or of the food quality within his territory, which may
confer honesty to his pheromonal signal and may explain why females
select the scent marked territories of these males.
Vitamin E (= α-tocopherol) is the main lipophilic antioxidant involved
in membrane defence (Brigelius-Flohe and Traber 1999). This vitamin is
of dietary origin and its defi ciency has severe pathological consequences,
such as infertility, neurological disorders and lung diseases. In the European
green lizard, Lacerta viridis, males show high proportions of tocopherol in
their femoral secretions (Kopena et al. 2009). These proportions increased
when males were experimentally fed supplementary vitamin E, and females
preferred to use areas scent-marked by these males with increased vitamin
E secretion levels (Kopena et al. 2011) (Fig. 3.6). This suggests that the cost of
Pheromones and Chemical Communication in Lizards 67
allocating vitamin E to secretions, diverting it from its important organismal
antioxidant function, may confer reliability to chemical signals of males.
Tocopherol may be the chemical signal directly used by females; however,
this antioxidant compound might simply increase duration and intensity
of information provided by other signaling compounds in secretions.
Similarly, linoleic and arachidonic acids are essential fatty acids that
mediate a wide range of physiological responses and maintain homeostasis,
and, in spite of their important functions, are also found in secretions of some
lizards in relatively high proportions (e.g., Martín et al. 2001). This suggests
a potential, but untested, signaling function of these essential fatty acids.
Reliability of chemical signals might be also based on other mechanisms.
For example, secretion of compounds could be differentially costly for
different individual males, because secretion depends on testosterone levels
and testosterone may have immunosuppressive effects (Folstad and Karter
1992; Belliure et al. 2004; Oppliger et al. 2004). Thus, only high quality males
could afford the trade-off of producing pheromones while avoiding the
detrimental effects of testosterone on their immune system. Female Podarcis
hispanica showed higher chemosensory responses to scents of males with
experimentally supplemented testosterone than to control scents (Martín
et al. 2007a), probably because production and concentration of glandular
secretions increased when testosterone increased. However, testosterone
also induced a decrease in proportion of cholesta-5,7-dien-3-ol, probably
as a consequence of its immunosuppressive effects (López et al. 2009b), and
Fig. 3.6 Strength of female preference for male scent in European green lizards (Lacerta viridis).
Female ‘strength of preference’ was calculated as the difference in the mean proportion of
females that were observed at the areas containing chemical cues from size matched vitamin
E supplemented vs. control males. Vitamin E difference is the difference in relative vitamin E
content of the femoral secretions of size-matched vitamin E supplemented vs. control males
within a male pair. From Kopena, R., Martín, J., López, P. and Herczeg, G. 2011. PLoS One
6: e19410. Figure 2.
68 Reproductive Biology and Phylogeny of Lizards and Tuatara
females preferred scent-marks of males that maintained high levels of this
compound in secretions independently of the experimental manipulation
(Martín et al. 2007a).
3.7 FUTURE DIRECTIONS
Although the roles of pheromones in lizard biology have been explored in
the studies cited above, there is still much work to do in several fi elds to
fully understand them. First, we need to know the compounds that may be
potentially used as pheromones. Many lizard species representing diverse
taxonomic lineages and geographical areas have abundant glandular
secretions that have never been chemically analyzed, even for species
in which glandular secretions are known to be important for chemical
communication. Moreover, most available information is from femoral or
precloacal glands, whereas almost nothing is known about chemicals from
cloacal secretions or feces that may have pheromonal functions. Different
compounds might be characteristics of some groups of lizards, but be absent
in others. The presence of different compounds might have a phylogenetic
effect, but it might also be linked to environmental conditions, or to the way
these chemicals are used in communication (e.g., as scent-marks on different
substrates, or as chemicals sampled directly from the individuals). This
emphasizes the need for more studies analyzing the chemical compounds
in glandular secretions of diverse groups of lizards from different habitats.
These descriptive studies will allow further comparative studies to clarify
the patterns of presence and abundance of compounds observed in each
species, and the causes (genetic and environmental) that explain the
evolution of gland secretions in lizards.
It is rarely known which specifi c compounds could act as pheromones.
After identification of compounds in glandular secretions, further
chemosensory tests should be conducted with the different compounds,
alone and combined in different proportions, to understand which
compounds are important in communication and how their variability in
abundance affects to the chemosensory responses of conspecifi cs.
The next step would be to identify the “message” of these chemical
compounds (i.e., the strategic element of the signal; Guilford and Dawkinds
1991). Behavioral tests are needed that relate differential responses to
differences in scents of different individuals for which chemical composition
of glandular secretions is known, and to differences in morphological
and physiological characteristics. We need to determine which traits are
suffi ciently important to be signaled in each species and how they are
Pheromones and Chemical Communication in Lizards 69
signaled through chemical cues. Also, lizards use other sensory systems in
communication, especially vision, and it may be important to understand
how information contained in multiple visual and chemical signals is
congruent, although sometimes used in different contexts, or different and
complementary, as well as the relative importance of one type of signal or
other in different species and environments.
Finally, we would need to analyze the mechanisms that allow the origin
and evolutionary persistence of a chemical signal, and how the environment
may drive the evolution of the chemical signaling system, especially
through changes in the design of the signal to maximize its effi cacy in
different environmental conditions (Guilford and Dawkinds 1991). This is
important, on the one hand, because if different populations of the same
species use different chemical signals for reproduction, this may later lead
to reproductive isolation between populations and speciation processes
(e.g., Gabirot et al. 2012a). On the other hand, current rapid human-induced
changes in the environment, such as global warming or contamination,
might affect the effi cacy of chemical signals with potential detrimental
consequences for reproduction and stability of populations (e.g., Martín
and López 2013b).
We encourage researchers to “look” for the roles of chemical signals
or pheromones in the social and reproductive behavior of lizards. Not
knowing these, we might be missing most of the information contained in
sexual displays and would fail to understand many fi eld observations of
ecology, behavior, and evolution, of these animals.
3.8 ACKNOWLEDGMENTS
We thank W. E. Cooper and A. Salvador for “inducing” us to study chemical
communication in lizards and for their very helpful comments, and to
all our friends that have worked with us for helping us to understand
pheromones of lizards and other reptiles: L. Amo, P. Aragón, C. Cabido,
A. M. Castilla, E. Civantos, M. Cuadrado, M. Gabirot, R. García-Roa, A.
Gonzalo, A. Ibáñez, R. Kopena, J. J. Luque-Larena, P. L. Moreira, J. Ortega, K.
M. Pilz, V. Pérez-Mellado and N. Polo-Cavia. We also thank “El Ventorrillo”
MNCN Field Station for the long term use of their facilities and Nino for
taking care of and feeding lizards and researchers. Financial support during
writing was provided by the project of Ministerio de Ciencia e Innovación
MICIIN-CGL2011-24150/BOS. The editors and reviewers are also thanked
for their input.
70 Reproductive Biology and Phylogeny of Lizards and Tuatara
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Pheromones and Chemical Communication in Lizards 75
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... In the case of lizards that use chemical communication, several possible chemical sources may be responsible for signal production. While there is some evidence that cloacal exudates and faecal pellets may contain socially relevant information, particularly the skin and the epidermal glands (generation and follicular) are considered important sources of chemical signals in lizard communication, enabling mate assessment, individual recognition, species recognition and sex identification (reviewed in [35][36][37][38][39]). Liolaemus tenius females, for instance, are more attracted to substrates covered with male epidermal gland secretions than to substrates scent-marked with male skin extracts [40]. ...
... using the presence of glandular or follicular epidermal glands as a proxy of chemical investment. Over the last two decades, studies of natural products chemistry combined with comprehensive behavioural assays have shown that the waxy secretions produced by epidermal glands are an important source of chemical signals for lizard chemical communication in a range of different clades [35,36,40,42]. Based on the most comprehensive literature search to date (n = 4341 lizard species), approximately 25% of all lizard species are equipped with follicular epidermal glands [42]. ...
... It is important to note that epidermal gland secretions are not the sole source of chemical information in lizards; faeces, cloacal secretions and skin lipids can contain socially relevant chemical stimuli too [36,39]. In other words, species that lack glands are not necessarily constrained by their ability to obtain chemical information [78]. ...
Article
The evolution of sociality and traits that correlate with, or predict, sociality, have been the focus of considerable recent study. In order to reduce the social conflict that ultimately comes with group living, and foster social tolerance, individuals need reliable information about group members and potential rivals. Chemical signals are one such source of information and are widely used in many animal taxa, including lizards. Here, we take a phylogenetic comparative approach to test the hypothesis that social grouping correlates with investment in chemical signalling. We used the presence of epidermal glands as a proxy of chemical investment and considered social grouping as the occurrence of social groups containing both adults and juveniles. Based on a dataset of 911 lizard species, our models strongly supported correlated evolution between social grouping and chemical signalling glands. The rate of transition towards social grouping from a background of 'epidermal glands present' was an order of a magnitude higher than from a background of 'no epidermal glands'. Our results highlight the potential importance of chemical signalling during the evolution of sociality and the need for more focused studies on the role of chemical communication in facilitating information transfer about individual and group identity, and ameliorating social conflict.
... Lizards (order Squamata) have both an olfactory epithelium and Jacobson's (vomeronasal) organ that discriminate a range of compounds and/or scents [7][8][9]. In addition, some species of lizards possess femoral glands (FGs), epidermal structures located on the ventral epidermis of the hind limbs that secrete a rich mixture of chemicals [10][11][12]. Femoral glands are usually dimorphic and, in general, more developed in males than in females [10,12,13]. Recent comparative analyses suggested that FGs and other epidermal glands may have played a role in the evolution of sociality in this vertebrate group [14]. ...
... In the case of sand lizards, steroids and fatty acids account for roughly 50% of the lipophilic fraction of femoral secretions [31]. Therefore, phospholipases and aldo-keto reductases could be involved in the metabolism or production of specific lipids in FGs, e.g., steroids and fatty acids, that could be further used in chemical signaling [11,22]. In addition, a fatty acid binding protein (FABP5; IR: 41) was found among the 667 highly expressed proteins. ...
Article
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Proteins are ubiquitous macromolecules that display a vast repertoire of chemical and enzymatic functions, making them suitable candidates for chemosignals, used in intraspecific communication. Proteins are present in the skin gland secretions of vertebrates but their identity, and especially, their functions, remain largely unknown. Many lizard species possess femoral glands, i.e., epidermal organs primarily involved in the production and secretion of chemosignals, playing a pivotal role in mate choice and intrasexual communication. The lipophilic fraction of femoral glands has been well studied in lizards. In contrast, proteins have been the focus of only a handful of investigations. Here, we identify and describe inter-individual expression patterns and the functionality of proteins present in femoral glands of male sand lizards (Lacerta agilis) by applying mass spectrometry-based proteomics. Our results show that the total number of proteins varied substantially among individuals. None of the identified femoral gland proteins could be directly linked to chemical communication in lizards, although this result hinges on protein annotation in databases in which squamate semiochemicals are poorly represented. In contrast to our expectations, the proteins consistently expressed across individuals were related to the immune system, antioxidant activity and lipid metabolism as their main functions, showing that proteins in reptilian epidermal glands may have other functions besides chemical communication. Interestingly, we found expression of the Major Histocompatibility Complex (MHC) among the multiple and diverse biological processes enriched in FGs, tentatively supporting a previous hypothesis that MHC was coopted for semiochemical function in sand lizards, specifically in mate recognition. Our study shows that mass spectrometry-based proteomics are a powerful tool for characterizing and deciphering the role of proteins secreted by skin glands in non-model vertebrates.
... A total of 16 compounds have been identified so far in the lipophilic fraction of S. jarrovii femoral gland secretions, most notably fatty acids (82%), salicylates (9%), and alkanes (4%) . Unlike some unsaturated fatty acids, saturated fatty acids do not elicit chemosensory responses from male conspecifics in lacertids (Martín and López 2014) nor in Sceloporus lizards (Romero-Diaz et al. 2020) and thus likely have a structural function. By embedding the compounds of interest (treatments 3-5) in a fatty-acid matrix that imitates the natural chemical background, we can investigate olfactory processes in a more ecologically relevant manner (Thomas-Danguin et al. 2014). ...
... Some studies in which signal design was manipulated at the elemental scale support this idea (e.g., Rand and Ryan 1981;Novotny et al. 1986;Hebets and Uetz 2000;Martins et al. 2005;Holveck and Riebel 2007). However, other studies have failed to detect significant effects of such changes or have found them to be limited to one or a few signal components, or component properties (e.g., Riebel 2009;Apps 2013;Martín and López 2014). These latter studies are consistent with signals relying on a few key elements, which have more weight than others (e.g., Micheletta et al. 2013), and thus, communication may only be affected when elemental changes involve those key elements directly. ...
Article
Full-text available
Most animal signals across sensory modalities are multicomponent traits that can be broken down into discrete elements. If different elements are perceived as unique, independent units (elemental perception), instead of as integrated percepts (configural perception), single changes in the presence/absence or the abundance of specific elements of a multicomponent signal may be enough to impact communication. Here, we found that male Yarrow’s spiny lizards (Sceloporus jarrovii) can discriminate single compounds of a multicomponent chemical signal (femoral gland secretions), different concentrations of a signaling compound, and a single compound from a mixture of compounds. In addition, one chemical compound elicited a response similar to that evoked by the complete natural scent. We conclude that perception of chemical signals in S. jarrovii lizards is elemental but also configural. The elemental perception of signaling compounds seems to occur with high sensitivity and narrow resolution, so that minor changes in single key elements may affect chemical communication. Given the multicomponent nature of most animal signals, hypotheses regarding signal function and evolution would be enhanced if researchers could determine whether these results apply to signals in other sensory modalities and identify the key elements of complex signals, from a receiver’s perspective. Significance statement Most signals in animal communication are quite complex. For example, odors are mixtures of multiple volatile chemical compounds, and the way in which receivers perceive and process these mixtures to extract relevant information influences the structure and evolution of chemical signals. In a series of behavioral trials, we investigated how male Sceloporus jarrovii lizards may perceive conspecific odors by testing their response to individual and combined mixtures of two compounds present in femoral gland secretions at two different concentrations. We demonstrate that lizards can discriminate structurally similar compounds and that the response to a compound changes when said compound is part of a larger mixture. Compound concentration affected the perception of individual compounds but not complex mixtures. Deciphering what elements and/or configurations are perceived in an odor mixture is the only way to understand the role of mixture composition and its impact on communication.
... As with many species in the genus, males of both C. indica and C. littoralis have femoral glands with each gland connected to the surface of the femoral or cloacal region via their associated pores. Femoral and pre-cloacal secretions are commonly used in intraspecific communication in geckos and other lizards (reviewed in Martín & López, 2014); and in at least one species of this genus, C. mysoriensis, are known to elicit intraspecific social interactions (Kabir et al., 2019). Therefore, we expect these chemical secretions to be signals in conspecific communication in other species of Cnemaspis as well. ...
... Visual traits may, hence, act as a heuristic (Marsh, 2002) for rapid "scan-and-act" decision making by other males, to assess the resource holding potential (RHP) of competitors at a distance before engaging in any agonistic interactions (Enquist & Leimar, 1983). Perhaps, the role of chemical secretions is more pronounced at a slightly later stage of social interaction, where it is imperative to better gauge the quality of the male (Martín et al., 2007;Martín & López, 2014;Mason & Parker, 2010), e.g., for similar-sized competitors. For example, in the closely related C. mysoriensis, females consistently select the chemical secretions of higher quality males over lower quality ones, in the absence of any visual cue, indicating chemical secretions in this species can signal health and performance indicators, such as ectoparasite loads and sprint speed (Joshi, 2020). ...
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Animal signals in multiple modalities expands the opportunity for effective communication. Among diurnal geckos of the genus Cnemaspis, chemical signalling traits preceded the evolution of visual traits. Males of all species possess chemical secreting ventral glands, but only in some species, males also express yellow gular patches. This difference in the expression of unimodal or multimodal signalling traits between closely related species provided us with an opportunity to understand the use of multimodal signals for communication. We studied receiver responses in Cnemaspis indica, a sexually monochromatic species, and in C. littoralis, a species where males possess yellow gulars. We performed behavioural trials where individuals of each species were exposed to only chemical stimuli, only visual stimuli, or both chemical and visual stimuli simultaneously from male and female conspecifics. Our results show that only chemical stimuli were necessary and sufficient to elicit responses in males and females of C. indica as well as in females of C. littoralis. However, males of the dimorphic C. littoralis required the multimodal stimulus to elicit movement-based responses. Our results suggest that the evolution of colour traits in diurnal geckos is associated with a partial shift in some receiver responses toward multimodal communication, with no addition to the behavioural repertoire.
... Recent decades have seen advances in some groups of vertebrates, e.g. mammals 14,15 , amphibians [16][17][18] and squamates 19,20 . However, studies focusing on other vertebrates such as birds and turtles remain scarce 21 . ...
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Despite the relevance of chemical communication in vertebrates, comparative examinations of macroevolutionary trends in chemical signaling systems are scarce. Many turtle and tortoise species are reliant on chemical signals to communicate in aquatic and terrestrial macrohabitats, and many of these species possess specialized integumentary organs, termed mental glands (MGs), involved in the production of chemosignals. We inferred the evolutionary history of MGs and tested the impact of macrohabitat on their evolution. Inference of ancestral states along a time-calibrated phylogeny revealed a single origin in the ancestor of the subclade Testudinoidea. Thus, MGs represent homologous structures in all descending lineages. We also inferred multiple independent losses of MGs in both terrestrial and aquatic clades. Although MGs first appeared in an aquatic turtle (the testudinoid ancestor), macrohabitat seems to have had little effect on MG presence or absence in descendants. Instead, we find clade-specific evolutionary trends, with some clades showing increased gland size and morphological complexity, whereas others exhibiting reduction or MG loss. In sister clades inhabiting similar ecological niches, contrasting patterns (loss vs. maintenance) may occur. We conclude that the multiple losses of MGs in turtle clades have not been influenced by macrohabitat and that other factors have affected MG evolution.
... Our conservative analysis revealed that two compounds were differently expressed between treatments. The tentative identification of the chemical compounds matches expectations since Lacertid lizards usually have alcohols in their secretions, and alcohols can be detected by conspecifics (Martín and López 2014). This modification of scent profile may have provided chemical cues for risks and triggered the observed behavioral response to conspecific scents. ...
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Organisms can gain information about predation risks from their parents, their own personal experience, and their conspecifics and adjust their behavior to alleviate these risks. These different sources of information can, however, provide conflicting information due to spatial and temporal variation of the environment. This raises the question of how these cues are integrated to produce adaptive antipredator behavior. We investigated how common lizards (Zootoca vivipara) adjust the use of conspecific cues about predation risk depending on whether the information is maternally or personally acquired. We experimentally manipulated the presence of predator scent in gestating mothers and their offspring in a full-crossed design. We then tested the consequences for social information use by monitoring offspring social response to conspecifics previously exposed to predator cues or not. Lizards were more attracted to the scent of conspecifics having experienced predation cues when they had themselves no personal information about predation risk. In contrast, they were more repulsed by conspecific scent when they had personally obtained information about predation risk. However, the addition of maternal information about predation risk canceled out this interactive effect between personal and social information: lizards were slightly more attracted to conspecific scent when these two sources of information about predation risk were in agreement. A chemical analysis of lizard scent revealed that exposure to predator cues modified the chemical composition of lizard scents, a change that might underlie lizards’ use of social information. Our results highlight the importance of considering multiple sources of information while studying antipredator defenses.
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Population isolation and concomitant genetic divergence, resulting in strong phylogeographic structure, is a core aspect of speciation initiation. If and how speciation then proceeds and ultimately completes depends on multiple factors that mediate reproductive isolation, including divergence in genomes, ecology, and mating traits. Here we explored these multiple dimensions in two young (Plio‐Pleistocene) species complexes of gekkonid lizards (Heteronotia) from the Kimberley–Victoria River regions of tropical Australia. Using mtDNA screening and exon capture phylogenomics, we show that the rock‐restricted H. planiceps exhibits exceptional fine‐scale phylogeographic structure compared to the co‐distributed habitat generalist H. binoei. This indicates pervasive population isolation and persistence in the rock‐specialist, and thus a high rate of speciation initiation across this geographically complex region, with levels of genomic divergence spanning the "grey zone" of speciation. Proximal lineages of H. planiceps were often separated by different rock substrates suggesting a potential role for ecological isolation; however, phylogenetic incongruence and historical introgression was inferred between one such pair. Eco‐morphological divergence among lineages within both H. planiceps and H. binoei was limited, except that limestone‐restricted lineages of H. planiceps tended to be larger than rock‐generalists. By contrast, among‐lineage divergence in the chemical composition of epidermal pore secretions (putative mating trait) exceeded ecomorphology in both complexes, but with less trait overlap among lineages in H. planiceps. This system — particularly the rock‐specialist H. planiceps — highlights the role of multidimensional divergence during incipient speciation, with divergence in genomes, ecomorphology, and chemical signals all at play at very fine spatial scales.
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The environment presents challenges to the transmission and detection of animal signalling systems, resulting in selective pressures that can drive signal divergence amongst populations in disparate environments. For chemical signals, climate is a potentially important selective force because factors such as temperature and moisture influence the persistence and detection of chemicals. We investigated an Australian lizard radiation (Heteronotia) to explore relationships between a sexually dimorphic chemical signalling trait (epidermal pore secretions) and two key climate variables: temperature and precipitation. We reconstructed the phylogeny of Heteronotia with exon capture phylogenomics, estimated phylogenetic signal in amongst-lineage chemical variation and assessed how chemical composition relates to temperature and precipitation using multivariate phylogenetic regressions. High estimates of phylogenetic signal indicate that the composition of epidermal pore secretions varies amongst lineages in a manner consistent with Brownian motion, although there are deviations to this, with stark divergences coinciding with two phylogenetic splits. Accounting for phylogenetic non-independence, we found that amongst-lineage chemical variation is associated with geographic variation in precipitation but not temperature. This contrasts somewhat with previous lizard studies, which have generally found an association between temperature and chemical composition. Our results suggest that geographic variation in precipitation can affect the evolution of chemical signalling traits, possibly influencing patterns of divergence amongst lineages and species.
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Chemical signals, such as those used in social communication, are often present as complex blends of compounds, suggesting that complexity is important in signal perception. Very few studies, however, have examined the interactions between different components of complex signals in social signalling. In the Mysore day gecko, Cnemaspis mysoriensis, secretions of males are sufficient to elicit a behavioural response in females and these male secretions differ from those of females in the presence of two key chemical compounds: cholesterol and squalene. This provided us with an opportunity to determine the functions and interactions of individual components in a complex multicomponent chemical signal. First, using tongue flick assays, we established that both components independently elicit a behavioural response in females, but not males. When presented as a multicomponent mix, the response levels of females were similar to those shown towards the individual components, thereby indicating that cholesterol and squalene are redundant components. Moreover, female responses towards these components matched their level of response towards natural male secretions, confirming that both cholesterol and squalene signal sex identity of males. When presented with a gradient of multicomponent stimulus concentrations, females, but not males, incrementally adjusted their tongue flick responses to different levels. Further, responses of females were similar regardless of whether cholesterol or squalene was at a higher relative concentration in the multicomponent stimulus. These last two sets of results indicate that the overall concentration, but not the relative ratio of cholesterol and squalene, has the potential to encode information about male quality. Lack of responses by males to these compounds across experiments strongly indicate the role of cholesterol and squalene in intersexual, and not intrasexual, communication. Overall, we show that two sex-specific compounds in a complex multicomponent chemical signal are effective in communicating complex sexual information from males to conspecific females.
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Experimental tests were conducted with the lizard Liolaemus tenuis (Tropiduridae), to determine the potential sources of pheromones used in its chemical communication, centered in the phenomenon of self-recognition. During the post-reproductive season, feces of both sexes and secretions of precloacal pores (present only in males) were tested. Stimuli were presented to lizards spread on rocks, and the number of tongue-flicks (TF) to the rocks was used as a bioassay to determine pheromone recognition. Feces contained pheromones involved in self-recognition, since lizards showed less TF confronted to rocks with suspensions of their own feces than with suspensions of feces of conspecifics or with water (control). In order to assess the chemical nature of self-recognition pheromones, feces were submitted to a sequential extraction with three solvents of increasing polarity, thereby obtaining three feces fractions. There were no differences in TF towards rocks with different fractions with own feces. Additionally, lizards showed similar TF to rocks with fractions of own and conspecific feces, suggesting that the separation procedure broke up a complex stimulus into parts that were not active individually as pheromones. Finally, males did not discriminate between precloacal secretions from themselves and from another male. It is possible that these secretions convey information relevant to or detectable by females only.
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Focusing exclusively on the chemically mediated interactions between vertebrates, including humans and other animals and plants, this monograph combines information from widely scattered technical literature in different disciplines. It will be an indispensable reference for undergraduates, graduate students and researchers interested in how chemical signals are used for inter- and intra-specific communication in vertebrates. © Cambridge University Press 2006 and Cambridge University Press, 2009.
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Agonistic behaviour in male lizards belonging to the fasciatus group of Eumeces (Scincidae) is directed primarily and perhaps almost exclusively to conspecific males. Heterospecific males, although visually quite similar to conspecific males, are usually ignored following chemosensory investigation by tongue-flicking. Male E. inexpectatus did not behave aggressively toward male E. fasciatus except when cloacal and skin odors of male E. inexpectatus had been transferred to them. Male E. inexpectatus therefore recognize conspecific males by a species-identifying odor. Eumeces fasciatus and E. laticeps may or may not possess similar abilities. -from Authors